So far, various printing plates for carrying out a planographic printing without using dampening water (hereafter, referred to as “waterless planographic printing”), in which a silicone rubber or fluororesin is used as ink repellent layer are proposed. The waterless planographic printing is a planographic printing method in which an image area and a non-image area are prepared in almost the same plane, and after depositing ink only on the image area by taking advantage of difference of ink depositability between the image area as ink receiving layer, and the non-image area as ink repellent layer, and printing is carried out by transferring the ink to a subject to be printed such as paper, and characterized by capability of printing without using dampening water.
As exposure method of the waterless planographic printing plate precursor, various methods are proposed, but it is roughly classified in system in which UV irradiation is carried out via an original image film, and computer-to-plate (CTP) system in which an image is directly written from a copy without using the original image film. As the CTP system, a method in which a laser light is irradiated, a method of writing by a thermal head, a method in which an electric voltage is partially charged by a pin electrode, a method in which ink repellent layer or ink receiving layer is formed by an ink jet, etc., are mentioned, but the method of using a laser light is more excellent than the other systems in view of resolution and printing plate making speed.
Furthermore, the waterless planographic printing plate precursor is roughly classified into photo(heat)sensitive layer removing type in which the photo(heat)sensitive layer is removed in exposure step or developing step and photo(heat)sensitive layer leaving type in which the photo(heat)sensitive layer is left even after the exposure step and developing step. Since the photo(heat)sensitive layer is removed in the photo(heat)sensitive layer removing type, by incorporating a coloring matter in the photo(heat)sensitive layer, it is possible to make a contrast between the image area and the non-image area. Accordingly, it has a merit that a plate inspection can be carried out even without a post-dyeing step. However, since cell depth which forms the image area is deep, a much amount of ink is needed at the time of printing. Since it is necessary to remove the photo(heat)sensitive layer in the depth direction, there is a problem that a fine image is hard to be reproduced.
On the other hand, a heat sensitive layer leaving type waterless CTP planographic printing plate precursor which has at least a heat sensitive layer and a silicone rubber layer on a substrate is proposed. In these printing plate precursors, since heat sensitive layer material is left in the laser irradiated portion also after the development, the amount of ink used at printing is small, and reproducibility of a fine image is also good. Since it can be exposed by a low laser output, not only advantageous in view points of running cost and laser life, but also a specific suction device becomes unnecessary, since abrasion sludge is not generated at the laser irradiation. However, since the heat sensitive layer leaves in both of the laser irradiated portion and the non-irradiated portion, it was difficult to make a color contrast of the laser irradiated portion and the non-irradiated portion, and a plate inspection was difficult.
In the production method of planographic printing plate by using the heat sensitive layer leaving type waterless CTP planographic printing plate precursor, a method having a step of dyeing the plate is proposed. In this method, although a plate inspection of the printing plate after dyeing the image area becomes possible, since an additional dyeing step becomes necessary, there were problems in handling a dyeing liquid, an upsizing of developing machine, cost, etc.
To this problem, a waterless planographic printing plate precursor containing a photofading material or photocoloring material in the silicone rubber layer and a waterless CTP planographic printing plate precursor containing a dye in the silicone rubber layer (for example, refer to JP 2002-244279 A (claims)) are proposed. These printing plate precursors can be inspected without the post-dyeing step. However, there was a problem that the plate having photofading material or photocoloring material cannot be handled in a light room. On the other hand, in the plate containing a dye, there were problems in coloring matter fixation in silicone rubber layer such that the dye in the silicone rubber layer causes an intralayer aggregation with a lapse of time, or the dye is concentrated in the heat sensitive layer interface of a higher polarity, or in the case where the silicone rubber layer has a protective film, the protective film absorbs the dye. Accordingly, there were cases in which the printing plate inspectability lowers, or an adhesion strength decrease between the silicone rubber layers and the heat sensitive layers due to the dye concentration. In addition, there are also cases in which the colored dye is extracted by various organic chemical liquids used for development or by a solvent in the ink used at the time of printing, and the printing plate inspectability lowers or a chemical liquid for development or the ink were contaminated with the extracted dye in some cases.
It could therefore be helpful to provide, in view of the above-mentioned problems, a waterless planographic printing plate precursor capable of a plate inspection without a post-dyeing step, capable of being handled in a light room and excellent in coloring matter fixing in the silicone rubber layer.